The first widely commercialized automobiles at the dawn of the last century were electric and powered by lead acid batteries. Lead acid batteries are currently used in cars for starting, lighting, and ignition purposes. Lead acid batteries cost, for example, about 170 dollars/kilowatt hour (kWh) and are cheaper than many other rechargeable batteries known. However, the energy content of lead acid batteries is rather low. The specific energy of lead acid batteries is, for example, about 35 watt hour (Wh)/kilogram (Kg) or about 20% of their theoretical value. This is notably reflected in the short driving range provided by the lead acid batteries, for example, of about 30 km in fully electric vehicles. A long recharge time, for example, of about 2 hours required for lead acid batteries necessitates in many applications, a cumbersome mechanical swap of a discharged battery by a charged battery.
By the year 1910, improvements in the performance of an internal combustion engine, the development of mechanical transmission, combined with a wide availability of liquid hydrocarbon fossils, resulted in the displacement of electric vehicles by gasoline vehicles in the terrestrial transportation market. Gasoline power systems provide high energy content, for example, about 4,000 Wh/kg at wheels, that is, about 500 kilometres driving range, and a quick mechanical refill. This provided gasoline power systems an advantage over batteries with solid electroactive materials (SEAM). Gasoline cars were widely used even through the oil crises of the 1970s. The oil crisis provoked a concern about the availability of hydrocarbon resources and promoted a short lasting interest in electric battery and hydrogen vehicles.
The current interest in electric cars started in 1990 with the passage of the zero-emissions vehicle mandate by the California Air Resources Board. Nickel-metal hydride batteries, commercialized around this time, were considered briefly for automotive applications. Although nickel-metal hydride batteries provided better performance than the lead acid batteries, for example, a driving range of about 60 km, a specific energy of about 60 Wh/Kg to about 90 Wh/Kg, an energy density of about 200 Wh/L-300 Wh/L, a specific power of about 200 W/kg, and an electric recharge of about 3 hours, albeit at a higher cost of about $1,000/kWh, the nickel-metal hydride batteries were not an acceptable replacement for gasoline from the customer's perspective.
By the year 1990, hydrogen fuelled polymer electrolyte membrane fuel cells (PEMFCs), which were originally developed within American and Soviet space exploration programs, became the leading contender among power sources for electric vehicles. The interest in PEMFCs was due to the following factors: the perceived availability of hydrogen fuel, a high specific energy, for example, of about 33.39 kWh/Kg for the low heating value of hydrogen (H2), a high specific power of PEMFCs, for example, about 0.7 W/cm2 at about 60% efficiency and about 0.35 kW/Kg and about 0.35 kW/L at the stack level, a competitive system energy density, for example, about 1,000 Wh/L for a 700 bar gas, and about 1200 Wh/L for 1 atmospheric pressure (atm) liquid H2 allowing for a 600 km driving range, as well as a good energy efficiency, for example, about 60% for PEMFCs versus about 13% for an internal combustion engine.
In the following 20 years, the idea of hydrogen economy and automotive fuel cells received a significant political and economic impetus which was justified by the concerns with the rising atmospheric carbon dioxide (CO2) levels and an unstable supply of liquid hydrocarbons. This was reflected in the statement by President G. W. Bush in his 2003 State of the Union address: “a child born today will be driving a car, as his or her first car, which will be powered by hydrogen and pollution free.”.” In 2004 General Motors was spending more than a quarter of its research budget on fuel cell vehicles and Larry Burns, GM's Vice President for R&D and Planning, said in February 2004 that the company will have a commercially viable fuel cell vehicle by 2010. In 2004, the State of California said it would build a hydrogen highway, with hydrogen fueling stations every 20 miles along major highways in the next few years. Despite the dedicated work of many scientists and engineers worldwide, the hydrogen fuelled polymer electrolyte membrane fuel cell (PEMFC) technology did not result in a market success of electric vehicles. The reasons are as follows: to achieve practically useful power density on the positive electrode, high platinum (Pt) loading is required which increases the cost of the PEMFCs; the dissolution of a Pt catalyst at positive potentials makes the positive electrode less durable; the lack of an inexpensive, sustainable, and a clean hydrogen source; and the lack of a hydrogen manufacturing and distribution infrastructure. Hence, there is a need for a technology that avoids the macro scale infrastructure required for hydrogen production and distribution and also reduces the amount of Pt required for on-board electricity generation.
Several revolutionary developments also occurred in the field of batteries with solid electroactive materials (SEAM). The advantages of a lithium (Li) metal anode, for example, a low equivalent weight, very negative redox potential, and a small cation size, allowing for an easy intercalation into cathode materials, were realized in the early 1970s. However, the first lithium batteries had a poor cycle life since the electronically insulating surface film formed on metallic lithium leads to dendritic Li plating during recharge. In 1981, researchers from Sony Corporation demonstrated a rechargeable lithium ion battery (LIB) with a graphite intercalation cathode. This lead to the commercialization of lithium batteries with a carbon anode in portable applications, within one decade. Since LIBs have a high energy density when compared to other commercialized room temperature batteries, LIBs have been used in commercial electric vehicles since the year 2010 despite a somewhat high cost, for example, of about $400/kW.
However, fully electric vehicles, unlike plug-in hybrids, based on lithium ion batteries (LIBs) did not achieve a widespread commercial success, primarily due to a low energy content, that is, a low driving range, and a high total cost of ownership of the batteries. For example, Nissan Leaf® of Nissan Jidosha Kabushiki Kaisha DBA Nissan Motor Co. Ltd., has a battery that weighs about 20% of the total car weight with about 200 Wh/Kg, that is, about 53% of the theoretical value, and about 230 Wh/L, and provides about 60 Km to about 100 Km driving range depending on whether the air conditioner is on or off. A larger sport utility vehicle (SUV), for example, Toyota RAV4® EV of Toyota Jidosha Kabushiki Kaisha TA Toyota Motor Corporation, also shows a similar performance. The often quoted statistics that 60% of daily car trips in the United States are less than 60 Km is apparently not helping the sales of lithium-ion battery powered cars as most drivers need the capability to make longer trips. Apart from the low driving range, the LIBs also have a low electric recharge rate, for example, the Nissan Leaf® takes about 30 minutes for a charge of about 80% of full capacity, and the construction of a large scale battery swapping infrastructure is not justified due to the lack of a sizable LIB electric vehicle market, as illustrated by a recent bankruptcy of Better Place. Also, the capital cost of the LIBs needs to be reduced in the long term, for example, from about $500/kWh to $125/kWh and from about $30/kW to $8/kW at 250 Wh/kg, 400 Wh/L, and 2 kW/kg.
The scientists at General Motors (GM) arrived at the same conclusion, that is, the battery electric vehicles based on current and targeted Li ion battery technology will be limited to small vehicle, low mileage-per-day applications due to relatively low specific energy and long recharge time constraints, and it is possible that fundamental physical limitations may prevent pure Li ion based battery electric vehicles (BEVs) from delivering the freedom of providing long trips, with intermittent quick refills, that consumers currently receive from their cars. According to Toyota spokesman John Hanson “We don't think that lithium-ion batteries are going to help us get to a point where we can dramatically increase volume and really call it a mass market. We're going to have a more significant breakthrough and probably go into some other area of battery chemistry.” MIT's Yet-Ming Chiang concurs: “It is clear that long-term vehicle electrification especially affordable 200 mile all-electric range—will require batteries with approximately three times greater energy densities at about one third the cost per kWh than that of LIBs.” Kevin See, analyst for Boston-based Lux Research, said “It is not realistic or feasible for automakers to significantly cut the price of lithium-ion batteries. There is going to be incremental improvement, but we don't believe it will be enough to spur the huge adjustment everyone was hoping for.” Tesla Motors has conceded that new technologies will eventually be required. According to Steve Visco, the founder of Polyplus: “What has happened over the past couple of years is the growing realization that lithium-ion chemistry will not take EVs to a mass adoption vehicle. It is just too expensive and they're too heavy.”
Numerous attempts to commercialize lithium ion batteries (LIBs) for use in fully electric vehicles in the last 5 years failed as eloquently illustrated by the mismatch of large production capacities and negligible sales by all 9 award recipients of the August 2009 $1.5 billion Department of Energy's (DOE) “Electric Vehicle Battery and Component Manufacturing Initiative” who had a primary focus on electric vehicle (EV) batteries including Dow Kokam, Johnson Controls, A123 Systems, Compact Power, EnerDel, General Motors, SAFT America, and LG Chem. The public's lack of appetite for battery-powered cars persuaded the Obama's administration in January of 2013 to back away from its aggressive goal to put 1 million electric cars on U.S. roads by 2015. According to Takeshi Uchiyamada, Toyota's Vice Chairman, “the current capabilities of electric vehicles”, based on fuel cells or lithium ion batteries, “do not meet society's needs, whether it may be the distance the cars can run, or the costs, or the long time to charge. Because of its shortcomings, that is, driving range, cost, and recharging time, the battery or fuel cell electric vehicle is not a viable replacement for most conventional cars. We need something entirely new”. Thus, there is a need for a solution that departs from the currently available technologies and differs from others under investigation in the electric vehicle battery field. More specifically, there is a need for a power source for electric vehicles that provides a longer driving range, lower total cost of ownership, and allows for a quick recharge or refill than lithium-ion batteries.
The history of technology teaches that if the show stopping part in any device is identified and replaced with another part, then this may change the device from a non-functional device to a functional device, though the performance in one or more parameters may have to be sacrificed. In the case of lithium batteries, the aforementioned abandonment of the metallic lithium electrode in favor of lithium intercalated into graphite resulted in about 30% decrease in the theoretical energy density but created a marketable battery with a long cycle life. Flow systems such as fuel cells (FCs) and redox flow batteries (RFBs) allow an independent scaling on energy and power, and are thus better suited for transportation than batteries with solid electroactive materials (SEAMs). Other advantages of flow systems, when compared to SEAM batteries, are a higher system energy density, if the reactants are not too dilute, a quick refill time, an intrinsic fluid heat management, and a simple cell balancing. The advantages of redox flow batteries over fuel cells are: electric regeneration that does not require a construction of a new fuel distribution infrastructure, for example, a hydrogen distribution infrastructure, higher efficiency, and in general, a lower cost. Conventional redox flow batteries such as vanadium redox flow batteries have a low energy density that translates into a short driving range, because the components have low solubilities and a large amount of an otherwise useless solvent which has to be carried on-board to keep the components in the fluid state. For this reason, flow batteries have been considered mostly for stationary storage applications rather than for electric vehicles.
A Massachusetts based start-up, 24M, proposed a method that retains the advantages of flow batteries while overcoming drawbacks of traditional solution chemistry, by developing a slurry flow battery based on the C6—LiFePO4 chemistry used by A123 Systems for batteries with solid electroactive materials (SEAM) or SEAM batteries. However, such a battery in an electric vehicle such as the Nissan Leaf® or the Toyota RAV4® would provide only from about 90 Km to about 150 Km driving range, even if the battery reaches, for example, about 80% of its theoretical energy density. Improvements in packing factor, that is the ratio of practical to theoretical energy density, by using, for example, binder free SEAM batteries with a soluble mediator or a soluble redox couple or metal containing ionic liquid flow batteries or protected Li metal anode, run into the fundamental limitation that the intrinsic energy densities of known battery chemistries are not sufficiently high for fully electric vehicle applications. Also, the cost of such batteries is likely to stay above the mid-term target of about $100/kWh and about $30/kW, or about $2,250/car with about 100 horsepower. Hence, there is an unmet need for flow batteries with higher energy content and a lower cost in order to gain market acceptance of fully electric vehicles.
Polymer electrolyte membrane (PEM) fuel cells have high power and energy density at low operating temperatures as well as a flow design which makes the PEM fuel cells well suited for automotive applications. Furthermore, fuel cells provide for a very high system energy density since the oxidant, that is, O2 is not carried on-board. However, the fundamental problems related to the slow kinetics of the oxygen electrode result in high cost and poor durability of PEM fuel cells due to the necessity of high Pt loading in the case of near ambient temperature fuel cells. Another problem with fuel cells, in general, is the source of the fuel, for example, hydrogen. Hence, there is a need for a discharge flow battery that ensures a high energy density, high energy efficiency, generates a high electric power by replacing the free oxygen from air with a high energy density and kinetically fast on-board fluid oxidant, and allows for the regeneration of a fuel and an oxidant from the exhaust products.
Flow batteries use electrochemical power cells similar to fuel cells. How batteries also use fluid reactants, for example, liquid, gaseous, or suspended reactants to store energy and to generate electric power. However, instead of oxygen or air, a different oxidant or a solution of an oxidant can be employed. Due to the carrying of an on-board oxidant, the flow battery typically entails a lower system energy density than a fuel cell. The reasons for using the on-board oxidant method comprise, for example, increasing the efficiency of energy conversion, reduction in the amount of precious metal catalysts, potential to change the operating temperature of the electrochemical power cell, improved heat management, the possibilities of electric recharge and of mechanical refill, etc. When compared to batteries with solid electroactive materials (SEAM) or SEAM batteries, for example, lithium ion batteries, flow batteries offer an independent scaling of energy and power, a higher ratio of practical to theoretical energy density that is, packing factor for systems with a sufficiently long discharge time, a possibility of quick mechanical recharge, intrinsic liquid cooling, etc. Commercialized redox flow batteries, such as Vanadium Redox Flow Batteries have low energy densities because of the use of redox couples with low solubilities and with a low number of redox-active electrons per electroactive atom. Paul Zigouras, Director of New Business Development at EPC Corporation, eloquently summarizes the status quo as: “Flow batteries are a great idea, but unfortunately, no fluid currently exists that will hold a decent amount of energy. Even the best experimental fluids have about ⅕th the energy density of the required value. I am hopeful, but also doubtful that a fluid will ever be developed that can effectively do this”.
Hydrogen-halogen flow batteries employ fluid reagents and products, and thus, may avoid the aforementioned energy density dilution by a solvent. In the series from fluorine (F2) to iodine (I2), the theoretical energy density decreases while the efficiency, cathode power, and exchange current increases. As a result, F2 has poor cycle efficiency, in addition to material compatibility issues, whereas I2 has a low energy density in addition to solubility problems. Hence, only bromine (Br2) and chlorine (Cl2) may be of interest for transportation applications. However, the chorine cells use an expensive ruthenium (Ru)-containing catalyst and provide poor energy efficiency. The theoretical energy density of hydrogen-bromine cells is only marginally better than that of lithium-ion batteries. The energy density becomes even lower if bromine is used as an aqueous solution with hydrogen bromide (HBr) to reduce the oxidant's cross over through membrane via the formation of Br3− anions and to lower the pressure of the Br2 vapour. Hydrogen-bromine cells are therefore considered at present mostly for grid storage rather than for electric vehicles.
There is a need for resolving the aforementioned TRIZ contradiction between energy density and energy efficiency of halogens, for example, by introducing a new dimension to the choice of oxidants, for example, by adding a second dimension of oxocompounds such as oxides and oxoacids to the one dimensional space of elements such as halogens. Although hydrogen-oxoacid flow batteries such as H2—HNO3 have been considered in the past, these flow batteries have poor discharge efficiency and lack the ability of electrical recharge or regeneration of the reagents. The direct electroreduction of halogen oxoacids is highly irreversible under the polymer electrolyte membrane fuel cell (PEMFC) conditions. There is a need to overcome this problem, for example, by performing a slow reduction of an oxocompound in a solution, that is, in three dimensions rather than on an electrode, that is, two dimensional.
Transition metal ion catalyzed electroreduction of oxoanions has been known for over 100 years. However, such reactions did not find applications in energy storage and conversion, mostly due to their poor reversibility. A more useful way to facilitate the electroreduction of halogen oxoanions is to employ a preceding homogeneous reaction such as comproportionation with a halide product as exemplary demonstrated for a halate by the equations below:XO3−+6e−+6H+=X−+3H2O on the electrode, slow.  (1)XO3−+5X−+6H+=3X2+3H2O in solution, fast.  (2)X2+2e−=2X− on the electrode, fast.  (3)
where X=Cl, Br, I.
In practice, reaction (3) may precede reaction (1) during the initial stage of the cycle. Furthermore, at high concentrations of halogen oxoanion and of an acid and for a thick diffusion layer, the steady-state limiting current, determined by the balance of the rate of halogen, that is, X2 intermediate formation via comproportionation (2) and by the rate of halogen loss into the solution bulk, can reach enormous values over 1 A/cm2.
The reverse process of oxidation of halides is generally believed to follow the same pathway. For example, the oxidation of the halides such as iodide, bromide, and chloride at alkaline pH shows that the reverse of the chemical reaction indicated by equation (2) occurs through the formation of an intermediate hypohalate via a homogeneous disproportionation: Here, R is a base:2X2+2H2O+2R−=2HXO+2X−+2RH  (4)followed by another homogeneous disproportionation:5HOX (hypohalous acid)=4X−+XO3−+H++2H2O  (5)or (4) and (5) combined3X2+3H2O+6R−=XO3−+5X−+6RH  (6)
Thus, disproportionation, for example, reaction (6), can be used to regenerate a halogen oxoanion from a halide present in the discharge fluid via an intermediate halogen produced by one or several routes of oxidation of halide.
The occurrence of homogeneous disproportionation reactions (4), (5), (6), and a comproportionation reaction (2) facilitates discharge and regeneration processes respectively in the energy cycle. The occurrence of these reactions allows for a high power, high efficiency operation based on a fast electrode reaction (X2+2e−=2X−) while performing slower steps such as reduction of the oxoanion with the electro-generated halide in the three dimensional bulk of the solution which can accommodate a higher reaction rate than the two dimensional electrode surface. Although the use of a mediator leads in theory to reduced energy efficiency compared to a direct electrode reaction, this thermodynamic loss of energy efficiency is often smaller than the kinetic loss associated with electrode over-voltage at the same power using oxidants such as oxygen or using direct electroreduction of the oxoanions.
The chemical methods of producing halogen oxoacids are used on an industrial scale. In the case of bromic acid, this chemical method consists of solution-phase disproportionation of bromine in Ba(OH)2, followed by Ba2+ precipitation with sulfuric acid and by evaporation of the excess water. However, this process irreversibly consumes Ba(OH)2, H2SO4 and generates BaSO4 waste. Also, this process does not co-produce a stoichiometric amount of hydrogen, which is required for the complete energy cycle of discharge and regeneration. Thus, this precipitation route does not meet the application requirements. An alternative method for preparing up to 40%-50% bromic acid via the electrooxidation of aqueous bromine solutions uses a lead dioxide anode at the current density of 10-20 mA/cm2 and a potential of +2.1 to +2.2 V versus a normal hydrogen electrode. Although this method is chemical and waste free, this method has poor energy efficiency and a low throughput.
Sunlight is a clean and carbon dioxide (CO2) free energy source and the sun's energy can be harvested thermally, photoelectrically, photochemically, or photoelectrochemically. While about 120,000 terawatts (TW) of sunlight, year averaged power, reaches the earth, the current total energy consumption of human civilization is only about 13 TW. Currently, with a wide scale utilization of solar technologies, there is a TRIZ contradiction between cost and efficiency intrinsic to all commercialized means of sunlight energy conversion. For example, semiconductor based photovoltaic solar panels, for example, polycrystalline silicon photovoltaic solar panels, multilayer photovoltaic solar panels, InxGa (1−x) Se2, etc., are either inefficient or too expensive. Photoelectrochemical water splitting into hydrogen (H2) and oxygen (O2) using anatase TiO2 nanoparticles also suffers from a low efficiency due to the high over voltage of the oxygen production centers. Hence, there is a need for a method for converting sunlight energy into chemical energy or electric energy at low cost and without producing any chemical waste.
Hence, there is a long felt but unresolved need for an electrochemical flow battery that provides for a high energy density, that is, a long driving range, a high energy efficiency and power at a low operational and manufacturing cost, and requires a short refill time. Moreover, there is a need for a method and a system that regenerates an oxidant and a fuel simultaneously from a discharge fluid, in stoichiometric amounts, without consumption of extra chemicals and without generating chemical waste and by using electric or solar energy as the primary energy source. Furthermore, there is a need for an electrochemical flow battery that provides better safety and stability by storing on-board and off-board a stable form of the oxidant.